Method of making basic refractories



Patented Apr. 15, 1941 METHOD OF MnKlN G BASIC REFRACTOBIES Max Y.Seaton, Greenwich, Conn., and Hugh B.

Hartzell, San Leandro, Call1'.,

amnion, by

mesne assignments, to Basic Dolomite, Incorporated, Cleveland, Ohio, acorporation of Ohio No Drawing. Original application May 23, 1935,

Serial No. 23,128. Divided and this application May 20, 1938, Serial No.209,140

4 Claims.

' This invention relates to method of making basic refractories; and itcomprises a method of producing heat-hardening and hardened basicrefractories wherein materials containing large proportions of magnesiaand small controlled proportions of'silica and of lime are heated to ahigh temperature for a sufllcient time to produce a crystallization ofsubstantially pure, granular magnesia in periclase form in a matrix ofhighly refractory silicate, the operation being advantageously in twostages. with an initial conversion of the greater part of the magnesiainto periclase enclosed in a matrix of monticellite, the iired materialbeing then comminuted, mixed with the proper proportion of lime toconvert the monticellite into dicalcium silicate, and the mixturere-fired to produce a refractory consisting of periclase in a matrix ofdicalcium silicate; all as more fully hereinafter set forth and asclaimed.

Most of the magnesia refractories used in melallurgical furnaces arestructurally weak; bricks, linings, etc., do not well withstand loads athigh temperatures and are disintegrated by alternate heating andcooling. To a certain extent, high refractory properties as regardstemperature are not consistent with good structural strength. Magnesiain a pure state is a weak material. It can be sintered and hardened byheat, the final result being crystallized or periclase magnesia, asubstance of considerable hardness and rigidity. But periclase in massis not a strong material.

We have found that dense, hard, refractory articles of good mechanicalstrength at high temperatures, can be made consisting of substantiallypure, granular magnesia in periclase form held in a matrix or hinder oflime and silica combined as calcium orthosilicate (dicalcium silicate);the silica-lime compound which has the highest fusing point of any ofthe calcium silicates.

The best combination of refractoriness and strength at high temperaturesappears to be afforded by a material containing about 80-85 per cent ofsubstantially pure, granular magnesia with a bonding material consistingof substantially pure, dicalcium silicate. To a certain extent, therelative proportions of matrix and periclase used in practice depend onthe composition and purity of the particular materials available and onthe details of firing. The proportion of matrix desirable variessomewhat with the particle size of the periclase. Sufficientrefractoriness and mechanical strength can be obtained over acomparatively wide range of weight ratios between the magnesia and thematrix. We have made good refractories in which the matrix was as littleas 5 per cent of the total while good-refractories have also beenproduced in which the matrix constituted more than 20 per cent of thetotal. Sufiicient refractoriness and good mechanical strength areobtained at either end of the range but we desire, ordinarily,compositions containing at least to per cent of included magnesia aspericlase.

In heating a system composed of magnesia in major amount and a smalleramount of silica and lime, in equimolecular proportions, it is foundthat magnesia, lime and silica unite to form a molten magma ofmonticellite (CaO.MgO.SiO2) as a liquid phase or eutectic includinggrains or crystals of magnesia of the nature of periclase. The size ofthe grain depends somewhat on the original particle size of the magnesiabut with prolonged heating, the grains grow somewhat in size. All themagnesia in the system above that required to form monticellite (with alittle MgO in solution) occurs as a disperse phase, as periclase. Bydoubling the proportion of lime, or by adding more lime to amonticellite magma. the result is difierent. The matrix now becomes amuch higher melting body of dicalcium silicate. Monticellite isdecomposed by lime added in the proper proportion, its MgO beingdisplaced by a second CaO molecule and the displaced MgO crystallizingout as periclase; the new periclase joining that already present.

In the present invention these principles are utilized. Puremonticellite with a little excess magnesia melts at 2768 F., and doesnot soften much below this point. If there be a little alumina present,as is often the case, the fusing point may drop to 2640" F. and thesoftening point to around 2400 F. The presence of small amounts of ironhas little effect on the fusing point or the softening point. A magma ofmonticellite with periclase becomes a plastic mass at the statedtemperatures. Dicalcium silicate or calcium orthosilicate (2CaO.SiO-2)with the amount of magnesia it will hold in solution has a melting pointof 3416" F., being a considerably more refractory substance. Again, thepresence of impurities may lower the melting point somewhat, but withreasonably pure materials, the melting point is not below about 3200 F.A magma of calcium orthosilicate and periclase becomes plastic at thestated temperatures, but

at all lower temperatures it is rigid and strong. In all cases, thepurer the matrix, the higheris the temperature of fusion and ofplasticity, respectively. The less the plasticity, the better is themechanical strength at high temperatures.

' We ordinarily endeavor to secure raw materials as pure as possible,the ideal being pure periclase in a matrix of pure orthosilicate, theorthosilicate used being at first monticellite and later ,converted intodicalcium silicate. The purer the materials, the higher is the softeningtempera ture. In this connection, however, we may note that a littleiron and alumina are permissible. A little iron for example, occurringas ferric oxid or as ferrous oxid, goes into the dispersed phase;

the former as dispersed crystallized magnesium qualities are lessened.Alumina present in more than small amounts is undesirable, as it lowersthe refractoriness. If any alumina be present, there should be providedenough excess lime to form tricalcium aluminate with all the alumina,over and above the lime required for the desired eutectic.

Dicalcium silicate or calcium orthosilicate, 2CaO.SiO2, exists inseveral physical forms. The terminology used as regards these issomewhat confused, but for the present purposes, the material producedand existing at a range of temperatures from about 2500 F. up may becalled the alpha form. The pure hot material in alpha form, goes overwithout change of volume, as the temperature lowers, into a beta form,stable at an intermediate range of temperatures (down to approximately1215 F.). The third modification, the gamma form, is stable at lowtemperatures; temperatures up to 1215 F. The beta form, however, in apure state, undergoes a sudden rearrangement to the gamma form oncooling to about 1215 F. This is accompanied by an increase in volumewhich may be as high as 10 per cent and generally results in shattering.It has been found, however, that the desirable forms -of dicalciumsilicate can be stabilized against passage into the undersirable gammaform by the presence of extremely small amounts of other bodies. Aluminais a stabilizer, but it is undesirable in the composition in anysubstantial amount and a much better stabilizer is P205 in mere traces;amounts which are of the order of those often found in ordinarylimestone. As little as one-half per cent of P205 calculated on the CaOis suflicient. P205 appears to stabilize the dicalcium silicate in thealpha or beta form; the material thus stabilized does not change withtemperature changes; and there is no tendency to shatter withtemperature changes.

While the direct production of periclase refractories with a eutectic ormatrix of dicalcium silicate in a single operation is possible, asdisclosed in our application Serial No. 23,128, of which thisapplication is a division, we have found that, as a matter of practice,it is better to use a twostage operation in which a matrix ofmonticellite with included periclase is first produced, and then thematrix is changed to dicalcium silicate. This requires two firings, butin the first firing the temperature required is lower and sufiicientopportunity is given for nearly all the magnesia in the mixture tocrystallize as periclase. There is but little of the total magnesia leftin the maing, in admixture with lime, may be shaped into the formdesired, bricks or granules, as the case may be, prior to the secondfiring. The second "firing may be accomplished by the heat of thefurnace in which the refractory is used. Granules, such as those used inlining basic open hearths, may be made of any size desired.

This embodiment we regard as the most desirable form of our inventionsince it enables the production of a heat-hardening basic refractory; amixture which can be shaped or placed, as the case may be, and whichunder the influence of heat will first soften, become plastic and thenbecome a unitary mass of higher melting point. A mixture of periclasebonded by monticellite, with a little lime, has these properties. Forlining basic open hearths, granular material containing monticellite maybe admixed with lime in the right proportion and put in place. Or, limemaybe mixed with fine ground monticellite-bonded periclase and themixture then granulated. In either event the granules become plastic,softening under the heat of the open hearth. Reaction takes place, withthe formation of a unitary dense mass wholly refractory at itstemperatures. Similarly, such a material may be tamped into place in afurnace wall, etc.

Basic open hearths are customarily provided with an acid roof; a roofcontaining silica as a predominating constituent. Roof and hearth areseparated by an intermediate course of neutral brick; often chromite.This practice is for the reason that no basic refractory has been knownheretofore which was sufilciently strong to warrant its use as a roof.Bricks made under the present invention with a matrix of dicalciumsilicate are amply strong for this purpose. They are not only strongenough at the temperatures encountered, but they are basic and inert. Nointermediate course of neutral bricks between roof and hearth isnecessary.

All natural magnesites contain more or less silica and lime, but with agood grade the amount of either may be small enough to requirecorrection by addition of further lime or further silica, either orboth. In going through the routine of first making a monticellite matrixand then converting this into a dicalcium silicate matrix, the amount oflime used in the first mix should be proportioned accordingly, and inmakin the second mix there is added a quantity of lime equal to or alittle more than that of the first mix. Magnesia precipitated inchemical processes, as in treating bitterns and brines with lime,usually contains a little lime but requires the addition of more. Silicamust be added. In adding lime, it is best to do it as milk of lime tosecure intimate admixture of the very small amount of lime.

Considerable difliculty is encountered if it is attempted to convert themagnesia, lime and silica to periclase bonded by calcium orthosilicatein a direct or one-stage process, because the refractorymass produced bydirect burning of the completely corrected raw material is so highlyrefractory that the particles do not sinter easily. The temperaturesrequired are too high for fuelfired furnaces; and moreover the sinteredmass is apt to be more or less porous. The higher the purity of thematerials, the more difllcult it is to obtain satisfactory results, andthe difficulties are;

especially desirable nor detrimental in the present process. In thefinished article, it occurs as stated: ferric oxid goes into thedisperse phase as crystallized magnesium ferrite; ferrous oxid occurs insolid solution in the periclase. Usually,

amounts of iron oxids below about one to three per cent have noappreciable effect either way as regards refractoriness in the presentinvention.

Alumina should be absent; but if present should not exceed amounts ofthe order of 0.5 per cent, with total fluxing oxides less than 1 percent.

Stabilized dicalcium silicate is in itself an excellent refractorymaterial, more basic than acid. But it is better from all points ofview, used as a matrix for granular material of higher melting points,such as crystallized periclase.

In an example of a specific embodiment of the invention, employing thetwo step treatment and producing aheat hardening material, the rawmaterial was a crude hydrated magnesia (magnesium hydroxid) having thefollowing analysis (on a dry basis) Parts by weight SiO'z .7 5 A1203-.30 F8203- .20 CaO .50 MgO 98.25

To 100 parts of this raw material were added 2.5 parts silica and 3.5parts lime (CaO), both in the form of fine powders, and the ingredientswere thoroughly mixed. The mix was fed to a rotary kiln and burned at ahigh temperature; around 3000 F. The resulting product had the followinganalysis:

Parts by weight S102 4.30 A1203 .40 F620: .30 CaO 5.30 MgO 89.70

Upon examination, the product was seen to have a matrix largely composedof monticellite, with periclase (in amount about 86.6 per cent byweight) as disperse phase. The product had a softening temperature ofaround 2800 F. and could readily be sintered into hard nodules.Conditions in the kiln were adjusted, however, to prevent anysubstantial fusion; the product was granular. The granule compositionwas then corrected by the addition of 3.35 parts lime as CaO per 100parts product. The mix was ground to a suitable grain size and thepulverized material was mixed with a temporary binder, dextrine in thiscase, to form unburnedbricks. These, upon subjection to strong heating,sintered and partially fused with the development of the desiredstructure; periclase in a matrix largely composed of calciumorthosilicate.- In addition to making into bricks, the granular materialwas adapted for use without pressing, as a furnace bottom material, toform a unitary lining.

In working with pure magnesias and magnesites, it is convenient to use anatural magnesium silicate of the talc and soapstone type, as a sourceof silica. Equilibrium is more quickly and readily reached than in usingsilica itself in the quartzose form of ordinary sand. Olivine issometimes useful.

Dolomitic limeused in the present invention adds both CaO and MgO to themix and can be used in introducing the calculated amount of CaO.

In the embodiment given, the materials used chanced to contain P205 invery small amount but sufficient to stabilize the dicalcium silicate. Inother cases using materials free of P205 in detectable traces, anysuitable phosphate, such as rock phosphate can be added; the amountsusually ranging between one and ten per cent P205 on the total lime.Also, various other compositions containing a total less than one percent of iron oxides and alumina and containing to per cent'MgO inpericlase form, with the balance substantially all dicalcium silicate,have been produced in accordance with our invention. In some instancesthe monticellite-periclas'e-lime composition was hardened by heating inplace; and in some instances bricks of this composition were firedbefore use to convert the monticellite and lime to calciumorthosilicate. In all cases, the final product had great strength andrefractoriness.

What we claim is:

1. The process of making magnesia refractories of high structuralstrength and great refractoriness which comprises heating magnesia withsilica and lime in sufficient proportion to form a eutectic ofmonticellite with crystallization of the magnesia as periclase, cooling,comminuting, mixing with enough fine material containing CaO to convertthe monticellite into calcium silicate and reheating to efiect thischange and produce a hard solid matrix of dicalcium silicate enclosingand bonding the crystallized periclase.

2. The process of claim 1 wherein the material containing CaO is causticlime.

CERTIFICATE OF CORRECTION. Patent No. 2,238,h28. I A r'11,1 191g.

MAX Y. ;SE A' ION, ET AL.

It is hereby certified that error ebpe ers in the printed specificationof the above numbered patent requiring correction as follows Page 5, aec- 0nd. column, line 51, claim 1, for 'calcium". reed --v dice.lci11mand that the said Letters. Batent should be read with this correctiontherein that the same may conform to the record ofthe case in the PatentOffice.

Signed and sealed this 20th day of May, A. D I

Henry Van Aredale (Seal) Acting Commissioner of Patents.

